1. Field of the Invention
The present invention relates in general to transfer devices for four wheel drive motor vehicles, and more particularly to transfer devices of a type which is equipped with an auxiliary transmission.
2. Description of the Prior Art
In order to clarify the task of the present invention, one conventional transfer device of the above-mentioned type will be described, which is disclosed in Japanese Patent First Provisional Publication No. 1-103540.
An engine torque outputted from a transmission is inputted to a center differential through an auxiliary transmission which can selectively assume low and high speed stages. At the center differential, the torque is distributed to front and rear wheel driving systems.
The auxiliary transmission is a planetary gear set which includes a sun gear, a pinion gear and a ring gear. A pinion carrier is used as output means for outputting the driving force which has been subjected to a speed change, and the sun gear or the pinion carrier is used as input means for inputting the driving force. Switching to the sun gear or the pinion carrier is carried out by a switch device.
The switch device comprises generally a sleeve which is axially movably splined to a hub formed about the input shaft. That is, by axially shifting the sleeve on the hub, the engagement of the sun gear or the pinion carrier is effected.
The center differential is a planetary gear set which comprises a sun gear, a pinion gear and a ring gear. The torque outputted from the auxiliary transmission is inputted to the pinion gear of the center differential through a pinion carrier, so that the torque for the front wheels is inputted to the sun gear and the torque for the rear wheels is inputted to the ring gear.
An output shaft for the rear wheels is arranged coaxial with a rotation axis of the center differential, and the output shaft for the front wheels is arranged in parallel with the rotation axis of the center differential. The output shaft for the rear wheels is united with the ring gear of the center differential, and the output shaft for the front wheels is connected to a differential switch device through a torque transmitting mechanism which includes a sprocket and a chain.
The differential switch device is so designed as to selectively engage or disengage the sun gear of the center differential, the ring gear of the same and the sprocket of the torque transmitting mechanism. That is, when the sun gear and the ring gear are engaged, the center differential becomes locked thereby to deliver the engine torque to only the rear wheel driving system, while, when the sun gear, the ring gear and the sprocket are engaged, the engine torque is distributed to both the front and rear wheel driving systems keeping the center differential locked. When the sun gear and the sprocket are engaged, the front and rear wheel driving systems are applied with torque which is provided by the differential function of the center differential.
The auxiliary transmission, the center differential, the differential switch device and the torque transmitting mechanism are supported by a common supporting shaft.
However, due to its inherent construction, the above-mentioned conventional transfer device has the following drawbacks.
That is, since the switch device of the auxiliary transmission is located independently between the input shaft and the planetary gear set, the axial length of the transfer device is inevitably increased. This induces lowering of the resonance point of the entire of the transfer device. Thus, it tends to occur that the transfer device is subjected to resonance even when the associated engine operates at normal operating speeds.
This phenomenon gives the vehicle passengers an uncomfortable riding feeling.